It is known that power factor correction (PFC) circuits are used as, for example, active filters for reducing harmonic distortion caused in a DC input/DC output power supply (DC-DC conversion circuit). However, because PFC circuits are not insulated from AC lines, there is a possibility that fatal accidents will occur, such as for example electrification, which may be caused when a human body touches electric currents between the output voltage of a PFC circuit and the ground, or such as electrical leakage between devices. Accordingly, DC-DC conversion circuits of an insulation type have conventionally been provided in a stage later than PFC circuits. However, DC-DC conversion circuits of an insulation type use transformers for transmitting electric power, resulting in a low efficiency (input power/output power) and complexity in circuit configurations.
Accordingly, it is desired that PFC circuits be insulated. However, because a transformer is used for insulation, a high surge voltage is caused by the leakage inductance of the transformer when the field effect transistor (FET) connected to the primary side of the transformer is turned off. Also, a high-voltage FET that can withstand the surge voltage has to be used, leading to a higher cost for FETs. Also, a high-voltage FET suffers from strong ON-resistance, which causes more losses and a reduction in the efficiency. Also, when a snubber circuit is used on the primary side of a transformer so as to suppress a surge voltage, resistors in the snubber circuit cause power losses, reducing the efficiency in DC-DC conversion circuits of an insulation type.
For example, a switching power supply device that reduces noise and switching losses caused by soft switching and that also regenerates, when a switching element is turned on, energy accumulated in a charge accumulation capacitor when the switching element is turned off is known. According to this switching power supply device, when a switching element is turned on, an auxiliary switching element is turned on first. Next, the auxiliary switching element is turned on during a period of one fourth of the resonance period between the capacitance of the charge accumulation capacitor and the inductance of the primary winding of the charge regeneration transformer so as to transmit the energy accumulated in the charge accumulation capacitor to the transformer. When the auxiliary switching element is turned off, the energy accumulated in the charge regeneration transformer flows into the DC input power supply via the charge regeneration diode. As a result of this, the entirety of the losses is reduced and higher frequencies are realized.
Also, for example, a transformer insulation DC-DC converter that includes a diode, a resonation capacitor, primary winding and an auxiliary capacitor of an auxiliary transformer, and secondary winding and a diode of an auxiliary transformer is known. The diode has its one terminal connected to the connection point between the first winding of the transformer and the transistor. The resonation capacitor is connected between the other terminal of the diode and the cathode terminal of the DC power supply. The primary winding and auxiliary transistor of the auxiliary transformer are connected in series between the anode terminal and the cathode terminal of the DC power supply. When the auxiliary transistor is turned on or turned off simultaneously with the transistor, and when the auxiliary transistor is turned on, the discharge energy of the resonance capacitor is regenerated to the DC power supply through the auxiliary transformer and the diode. As a result of this, the switching losses and noise in the transformer insulation DC-DC converter are reduced and the efficiency is improved.
Patent Document 1: Japanese Laid-open Patent Publication No. 11-178341
Patent Document 2: Japanese Laid-open Patent Publication No. 11-318075